Sounding Reference Signal (SRS) is a signal which is used to measure channel state information (SCI) between a user equipment (UE) and an e-Node-B (eNB). In the long term evolution (LTE) system, the UE regularly sends the uplink SNR according to the parameters such as bandwidth, frequency domain position, period and subframe offset indicated by the eNB. The eNB judges the uplink CSI of the UE according to the received SRS and performs the frequency domain selecting scheduling and close loop power control according to the acquired CSI.
In the LTE system, the SRS sequence sent by the UE is acquired by cyclically shifting a root sequence ru,v(n) by α in time domain. Different SRS sequences can be acquired by cyclically shifting the same root sequence by different αs, and the acquired SRS sequences are orthogonal with each other, therefore, these SRS sequences can be allocated to different UEs to implement Code Division Multiple Access among the UEs. In the LTE system, the SRS sequence defines eight cyclic shifts indicated with a 3-bit signaling and respectively being 0, 1, 2, 3, 4, 5, 6 and 7. In other words, in the same time-frequency resources, the UE within the cell has eight available code resources, the eNB can configure up to eight UEs with the same time-frequency resources to send SRS at the same time.
In the LTE system, the tree structure is used to configure the frequency domain bandwidth of the SRS. Each SRS bandwidth configuration corresponds to one tree structure, and the highest-level SRS bandwidth corresponds to the maximum SRS bandwidth, or called the SRS bandwidth range of this SRS bandwidth configuration, Tables 1 to 4 show the SRS bandwidth configuration in different uplink SRS bandwidth ranges. Take the SRS bandwidth configuration indexed by 1 in Table 1 for example, BSRS=0 is the level 0 and is the highest level in the tree structure, the SRS bandwidth of this level is the bandwidth corresponding to 32 Resource Blocks (RBs) and is the maximum SRS bandwidth of this SRS bandwidth configuration; BSRS=1 is the level 1, the SRS bandwidth of this level is the bandwidth corresponding to 16 RBs, and one SRS bandwidth of its upper level, level 0, is divided into two SRS bandwidths of the level 1; BSRS=2 is the level 2, the SRS bandwidth of this level is the bandwidth corresponding to 8 RBs, and one SRS bandwidth of its upper level, level 1, is divided into two SRS bandwidths of the level 2; BSRS=3 is the level 3, the SRS bandwidth of this level is the bandwidth corresponding to 4 RBs, and one SRS bandwidth of its upper level, level 2, is divided into two SRS bandwidths of the level 3, the tree structure is shown as FIG. 1. mSRS,b in Table 1 denotes the SRS bandwidth and Nb denotes the number of the blocks divided from the upper level.
In addition, in the same SRS bandwidth, subcarriers of the SRS are placed with a certain interval, that is, the transmission of the SRS applies the comb structure, wherein the number of frequency combs is 2. As shown in FIG. 2, when each UE sends the SRS, it only uses one (Comb=0 or Comb=1) of the two frequency combs, therefore, the UE can only use a sub-carrier whose frequency domain index is even or odd number to transmit the SRS. This comb structure allows more UEs transmitting the same SRS in the same SRS bandwidth.
In the LTE system, the eNB first allocates a bandwidth configuration index CSRS to all UEs in the cell, according to the number of RBs (NRBUL) corresponding to the current uplink bandwidth, it can be determined that which one of tables 1-4 would be used, and then according to CSRS, the SRS bandwidth configuration used by the current cell can be determined. For some UE, the eNB might allocate one SRS bandwidth index BSRS (the index of the level in which the UE is located) to the UE. The UE can acquire the SRS bandwidth used by it according to the SRS bandwidth configuration in the cell and the SRS bandwidth index BSRS. For example, if the SRS bandwidth configuration index of the current cell is CSRS=1, NRBUL=50, the SRS bandwidth configuration of the current cell is shown in the second row in Table 2. If the SRS bandwidth index allocated by the current cell to the UE is 1, the SRS bandwidth of this UE occupies 16 RBs, and the SRS bandwidth of the UE is within the range of the SRS bandwidth (that is, the range of the maximum SRS bandwidth, 48 RBs).
After the UE acquires its own SRS bandwidth, it determines the frequency domain initial position at which the UE itself sends the SRS according to the frequency domain position nRRC in the uplink signaling sent by the eNB. As shown in FIG. 3, the UE allocated with different nRRC sends the SRS in different areas within the range of the cell SRS bandwidth.
TABLE 1(6 ≦ NRBUL ≦ 40)SRS bandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthconfigurationBSRS = 0BSRS = 1BSRS = 2BSRS = 3CSRSmSRS, 0N0mSRS, 1N1mSRS, 2N2mSRS, 3N303611234341132116282422241464141320145414141614441415121434141681424141741414141
TABLE 2(40 < NRBUL ≦ 60)SRS bandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthconfigurationBSRS = 0BSRS = 1BSRS = 2BSRS = 3CSRSmSRS, 0N0mSRS, 1N1mSRS, 2N2mSRS, 3N304812421224314811638242240120245413361123434143211628242524146414162014541417161444141
TABLE 3(60 < NRBUL ≦ 80)SRS bandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthconfigurationBSRS = 0BSRS = 1BSRS = 2BSRS = 3CSRSmSRS, 0N0mSRS, 1N1mSRS, 2N2mSRS, 3N30721243122431641322162442601203454134812421224344811638242540120245416361123434173211628242
TABLE 4(80 < NRBUL ≦ 110)SRS bandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthSRS-BandwidthconfigurationBSRS = 0BSRS = 1BSRS = 2BSRS = 3CSRSmSRS, 0N0mSRS, 1N1mSRS, 2N2mSRS, 3N30961482242461961323162442801402202453721243122434641322162445601203454164812421224374811638242
In the LTE system, the UE only sends the SRS in the last single-carrier frequency-division multiple access (SC-FDMA) symbol of the subframe in time domain. The configuration of the UE transmitting the SRS in time domain is related to four parameters: cell-specific SRS period TSFC and the subframe offset ΔSFC, as well as the UE-specific SRS period TSRS and the subframe offset Toffset. Table 5 and Table 6 respectively show the cell-specific SRS periods and subframe offsets in the frequency division duplexing (FDD) system and the time division duplexing (TDD) system, and the cell-specific SRS period and the subframe offset indicate the time domain subframe position by which all possible UEs in the cell transmit the SRS, while in other subframes, the use of the last SC-FDMA symbol has no relationship with the SRS transmission. For example, the eighth row in Table 5 is srsSubframeConfiguration=7, as shown in FIG. 4, the corresponding TSFC=5, and the ΔSFC={0, 1}, the cell-specific SRS period in the cell is 5 subframes, and the positions of the subframe 0 and the subframe 1 in each period can be used by the UE to send the SRS. “S” in FIG. 4 denotes the subframe with the eNB being configured with SRS resource.
TABLE 5sounding reference signal subframe configuration in FDDConfigurationPeriodTransmissionTSFCoffsetsrsSubframeConfigurationBinary(subframes)ΔSFC (subframes)000001{0}100012{0}200102{1}300115{0}401005{1}501015{2}601105{3}701115{0, 1}810005{2, 3}9100110{0}10101010{1}11101110{2}12110010{3}13110110{0, 1, 2, 3, 4, 6,8}14111010{0, 1, 2, 3, 4, 5,6, 8}151111reservedreserved
TABLE 6sounding reference signal subframe configuration in TDDConfigurationPeriodTransmissionTSFCoffset ΔSFCsrsSubframeConfigurationBinary(subframes)(subframes)000005{1}100015{1, 2}200105{1, 3}300115{1, 4}401005{1, 2, 3}501015{1, 2, 4}601105{1, 3, 4}701115{1, 2, 3, 4}8100010{1, 2, 6}9100110{1, 3, 6}10101010{1, 6, 7}11101110{1, 2, 6, 8}12110010{1, 3, 6, 9}13110110{1, 4, 6, 7}141110reservedreserved151111reservedreserved
Table 7 gives the UE-specific SRS periods and subframe offsets in the FDD system, and Table 8 gives the UE-specific SRS periods and subframe offsets in the TDD system. The UE-specific SRS period and subframe offset provide the time domain period and subframe position by which some UE transmits the SRS. Take ISRS=17 in Table 7 as an example, as shown in FIG. 5, the UE sends one SRS every 20 ms, and its time domain position is in the first subframe within 20 ms. “S” in FIG. 5 denotes the subframe at which the UE sends the SRS.
TABLE 7UE specific SRS Period TSRS and Subframe Offset Configuration Toffsetin FDDSRS ConfigurationIndex ISRSSRS Period TSRS (ms)SRS Subframe Offset Toffset0-12ISRS2-65ISRS − 2 7-1610ISRS − 717-3620ISRS − 1737-7640ISRS − 37 77-15680ISRS − 77157-316160ISRS − 157317-636320ISRS − 317 637-1023reservedreserved
TABLE 8UE specific SRS Period TSRS and Subframe Offset Configuration Toffsetin TDDSRSConfiguration Index ISRSSRS Period TSRS (ms)Subframe Offset Toffset020, 1120, 2221, 2320, 3421, 3520, 4621, 4722, 3822, 4923, 410-145ISRS − 1015-2410ISRS − 1525-4420ISRS − 2545-8440ISRS − 45 85-16480ISRS − 85165-324160ISRS − 165325-644320ISRS − 325 645-1023reservedreserved
Single User Multiple Input Multiple Output (SU-MIMO) means that one UE is configured with multiple transmitting antennas to transmit information simultaneously, while the eNB is configured with multiple receiving antennas to receive the information simultaneously. The LTE system does not support the uplink SU-MIMO, therefore, the UE only has one antenna at each time point to send the SRS. To prevent time fading, the UE in the LTE system is configured with two transmitting antennas to support the antenna selection. When the antenna of some UE is selected to be enabled, the UE can select the antenna which is used to transmit the SRS according to nSRS at different time. When the frequency hopping of the SRS in the frequency domain is not enabled, the equation for calculating the antenna index α(nSRS) is:α(nSRS)=nSRS mod 2;
When the frequency hopping of the SRS in the frequency domain is enabled, the equation for calculating the antenna index α(nSRS) is:
      a    ⁡          (              n        SRS            )        =      {                                                                                        (                                                            n                      SRS                                        +                                          ⌊                                                                        n                          SRS                                                /                        2                                            ⌋                                        +                                          β                      ·                                              ⌊                                                                              n                            SRS                                                    /                          K                                                ⌋                                                                              )                                ⁢                mod                ⁢                                                                  ⁢                2                                                                    when                ⁢                                                                  ⁢                K                ⁢                                                                  ⁢                is                ⁢                                                                  ⁢                even                                                                                                          n                  SRS                                ⁢                mod                ⁢                                                                  ⁢                2                                                                                      when                  ⁢                                                                          ⁢                  K                  ⁢                                                                          ⁢                  is                  ⁢                                                                          ⁢                  odd                                ,                                                    ⁢                                  ⁢                                  ⁢        where        ⁢                                  ⁢                                  ⁢        β            =              {                                            1                                                                        where                  ⁢                                                                          ⁢                  K                  ⁢                                                                          ⁢                  mod                  ⁢                                                                          ⁢                  4                                =                0                                                                        0                                                      otherwise                .                                                        
The Further Advancements for E-UTRA (LTE-Advanced) system is the evolved version of the LTE system. Besides satisfying or over-satisfying all relevant requirements in the 3GPP TR 25.913: “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)”, it also satisfies or beyond the IMT-Advanced requirements proposed by the ITU-R. In the LTE-A system, the SU-MIMO is supported in the uplink system, and at most four antennas can be used as the uplink transmitting antennas, that is to say, the UE can use multiple antennas to send SRS at the same time, while the eNB needs to estimate the condition of each channel according to the SRS received in each antenna.
When transmitting the SRS with multiple antennas, the UE needs to configure orthogonal resources for each transmitting antenna in order to acquire the accurate channel estimation, and the orthogonal resources can be time domain resources, frequency domain resources and code resources. Therefore, since each UE needs to be configured with several orthogonal resources when there are multiple antennas, compared with the LTE system, the resource overhead for sending the SRS in the LTE-A system is larger, which has two aspects of influence, one aspect is: the increase of the SRS resource overhead will reduce the resources for bearing the service information, thus affect the system efficiency; the other aspect is: since the amount of the resource for sending the SRS is fixed, the increase of the SRS resource overhead of a single UE increases, thus the number of UEs which can be accommodated in the system is reduced.
Furthermore, when the LTE-A applies asymmetrical resources, the downlink has broader bandwidth than that of the uplink, which on the other hand makes the uplink channel more crowded and results in the higher requirements by the system for the utilization efficiency of the uplink resources.
Therefore, how to reduce the SRS overhead when there are multiple antennas is a problem demanding prompt solution.